In recent years, for the purpose of attaining reduced weight and enhanced flexibility of a function element such as a semiconductor element, an MEMS element and a display element, the technological development for forming these elements on a polymeric film has been actively conducted.
It has been said that, for forming a function element such as a semiconductor element, an MEMS element and a display element on the polymeric film surface, processing by a so-called roll-to-roll process utilizing the flexibility which is one of characteristics of the polymeric film is ideal. However, in the semiconductor industry, MEMS industry and display industry, a process technique suitable for a wafer-based or glass substrate-based rigid planar substrate has been constructed. As a practical option, by bonding a polymeric film to a rigid support made from an inorganic substance such as a metal plate, a wafer and a glass substrate and peeling off the film from the support after forming a desired element, it is possible to obtain a function element formed on the polymeric film utilizing the existing infrastructure. With regard to the polymeric film used at the time of bonding the polymeric film to the support made from an inorganic substance, the surface smoothness, dimensional stability, cleanness, resistance to process temperature, and resistance to a chemical liquid used in fine processing which are at a level sufficient for preventing troubles in forming the function element are required.
In a thin film transistor element and the like, the surface smoothness has been known to largely affect the element performance. Moreover, in the case where there is a scratch or the like on the surface, the case is not desirable because the scratch or the like causes disconnection of wiring.
With regard to the dimensional stability, among semiconductor thin films. Si has a coefficient of linear thermal expansion of approximately 3 ppm/° C., and in the case where this thin film is deposited on a substrate, when the difference in the coefficient of linear thermal expansion between the substrate and the thin film is large, the stress accumulated in the thin film causes the deterioration in performance, and the warpage and exfoliation of the thin film. Particularly, in the case where the thin film is heated at a high temperature during the film forming process, it follows that the stress caused by the difference in the coefficient of linear thermal expansion between the substrate and the thin film increases with a change in temperature. With regard to the resistance to process temperature, in the formation of a low-temperature polysilicon thin film transistor, there is a case where a treatment of heating at 450° C. for 2 hours is required in a dehydrogenation process. Moreover, for forming a hydrogenated amorphous silicon thin film, there is a possibility that a substrate is heated at a temperature of approximately 200° C. to 300° C. In this case, even when a thermoplastic resin is used as the polymeric film, the performance is unsatisfactory.
With regard to the resistance to a chemical liquid used in fine processing, since a chemical liquid treatment including an acid treatment and an alkali treatment, a vacuum step of forming a thin film, and the like are included in a semiconductor process in which the resist coating, exposure, etching and resist peeling are repeated, the resistance to a chemical liquid used therein is required.
As the material of a base material of an electronic component such as information and communication equipment (broadcast equipment, mobile radio equipment, portable communication equipment, and the like), radar equipment and a high speed information processor, ceramic has hitherto been used. The base material made from ceramic has heat resistance and is adaptable to the heightened frequency (up to GHz band) in the signal band of the information and communication equipment in recent years. However, since ceramic is not flexible and cannot be made thin, the applicable field is restricted.
On that account, investigations have been conducted for the purpose of using a polymeric film made from an organic material as a base material of an electronic component, and there have been proposed a polymeric film made from a polymer such as polyethylene naphthalate and polyethylene terephthalate, a film made from a polyimide, a film made from polytetrafluoroethylene, and glass fiber-reinforced epoxies. The film made from a polyimide is excellent in heat resistance and is tough, and therefore the film has a merit that the polymeric film can be made thin.
Since these polyimide films generally have a large coefficient of linear thermal expansion, and have problems that the dimensional change caused by the change in temperature is significant and these films are not suitable for the production of a circuit having fine wiring, the applicable field is restricted. As described above, a device including a polyimide film with physical properties satisfactory for a base material provided with heat resistance, high mechanical properties and flexibility has not yet been obtained.
As a polyimide film with a heightened tensile elastic modulus, there has been proposed a polyimide benzoxazole film made from a polyimide having a benzoxazole ring in the main chain (see PTD 1). There has also been proposed a printed wiring board including the polyimide benzoxazole film as a dielectric layer (see PTD 2 and PTD 3). Although these polyimide benzoxazole films made from a polyimide having a benzoxazole ring in the main chain have an improved tensile breaking strength, an improved tensile elastic modulus, and a satisfactory coefficient of linear thermal expansion, in contrast to their excellent mechanical properties, the thinner the film is, the more difficult it is to be handled. Accordingly, these films still have problems that they are insufficient from mechanical and dynamical aspects, and the like.
Although a polyimide having a biphenyl structure in the main chain also has an improved tensile breaking strength, an improved tensile elastic modulus, and a satisfactory coefficient of linear thermal expansion, in contrast to its excellent mechanical properties, the thinner the polyimide film is, the more difficult it is to be handled. Accordingly, the polyimide film still has problems that it is insufficient from mechanical and dynamical aspects, and the like.
Bonding a polymeric film to an inorganic substrate using a tackiness agent or an adhesive agent and processing the layered product have been heretofore widely performed (PTD 4). However, with regard to polysilicon, an oxide semiconductor, and the like, in the case where a process in a temperature range of approximately 200 to 500° C. is required, no method is known in which an adhesive agent or a tackiness agent for bonding use having a sufficient resistance for practical use is employed. Moreover, since the conventional thermoplastic resin generally has a large coefficient of linear thermal expansion and there is a limit for making this layer thin, as compared to a polyimide film and a substrate made from an inorganic substance, an adhesive layer made of a thermoplastic resin and the like has a tendency to adversely affect the dimensional stability in a heating process.
As a procedure for attaining the heat resistance, there has been disclosed a procedure of performing a step of forming a resin substrate on a fixed substrate with an amorphous silicon film constituting a peeling layer interposed therebetween, a step of forming at least a TFT element on the resin substrate, and a step of irradiating the amorphous silicon film with a laser beam and then peeling off the resin substrate along the amorphous silicon film from the fixed substrate to prepare a display device having flexibility by using the resin substrate (PTD 5). However, at the time of peeling, since laser radiation to an adhesive agent layer is performed and etching means is used, the process is complicated and requires high production costs. Bonding two polymeric films together by UV irradiation has been disclosed (PTD 6), and it has been described that using a coupling agent in this case is also effective, but the case only relates to bonding between two polymeric films and does not relate to performing the adhesion/peel force control relating to the coupling agent itself by UV light irradiation.
In the case of a device in which the film is required to have surface smoothness, in general, as compared to the surface of a glass substrate, a film containing a slip agent has large surface roughness. Therefore, the film excellent in productivity is difficult to be used for the device. Moreover, in the case where a film free from a slip agent is transported by many rolls, the film inevitably suffers from fine scratches during the process. As such, except to apply a varnish free from a slip agent directly to a glass substrate or the like and bake the coating film, it has been difficult to prepare a smooth surface.